1. Field of the Invention
This invention relates to fuel cells, and more particularly to gas seals for fuel cells.
2. Description of the Prior Art
In a fuel cell, a matrix layer filled with electrolyte is sandwiched between a pair of electrodes. Each electrode comprises a substrate with a thin layer of catalyst disposed on the surface thereof facing the electrolyte. Each electrode substrate is constructed to permit a reactant gas (generally either air or hydrogen) to pass therethrough and contact the catalyst. This is the gas diffusion type of electrode. A common characteristic of all fuel cells is the necessity for preventing leakage and inadvertent mixing of the reactant gases both within and external to the cell. Since the electrode substrates (and certain other components of the fuel cell stack) are gas porous, means must be provided for preventing "in-plane" gas leakage from the edges of these substrates.
One type of edge seal is described in commonly owned U.S. Pat. No. 3,867,206 Trociolla et al. The key to that invention comprises altering the characteristics of the ends or periphery of the electrode substrates such that they can be saturated with a liquid and such that they will remain saturated with the liquid throughout operation of the cell. The liquid is held in the edge by capillary action and forms a barrier which prevents gas from escaping through the otherwise porous material. The liquid also forms a seal against the surface of adjacent components thereby preventing gas from escaping between the contacting surfaces of these components. Prior art electrodes have a typical mean pore size of 40-80 microns. This pore size is too large for the edges to hold electrolyte with sufficiently high capillary force to provide a satisfactory seal. The hereinabove referred to Trociolla et al patent teaches reducing the pore size along the edges by impregnating the edges with a hydrophilic material. Commonly owned U.S. Pat. No. 4,035,551 teaches impregnating the edge portions with the same material from which the electrolyte retaining matrix is made (col. 7, lines 4-19). Impregnation may be accomplished by forming an aqueous dispersion of the impregnating material; screen printing this "ink" onto the edge; and removing the liquid carrier by heating.
In the past we have successfully impregnated the edges of 0.010-0.020 inch thick substrates with an inert material such as silicon carbide. However, with the advent of ribbed substrates such as shown and described in commonly owned U.S. Pat. No. 4,115,627, uniform and adequate impregnation of the edges has not been possible using prior art techniques because the thickness of the ribbed substrates is 5 to 7 times greater than the nonribbed substrates of the prior art. The problem is additionally aggravated because present substrate pore sizes are only between 20 and 40 microns, which is much smaller than those of the prior art making impregnation more difficult. It has thus not been possible to obtain a uniform and adequate impregnation of, for example, silicon carbide within these edges. Using prior art techniques, our best effort with a 0.080 inch thick substrate having an initial pore size range of 28 to 43 microns has been to reduce the pore size to the range of 1.5 to 36.6 microns. These seals were not able to meet our leakage requirement of less than 1.0.times.10.sup.-5 lbs. N.sub.2 /hr./inch of seal at a pressure drop of 4.0 inches of water.